Electrophotography is a useful process for printing images on a receiver (or “imaging substrate”), such as a piece or sheet of paper or another planar medium, plastic, glass, fabric, metal, or other objects as will be described below. In this process, an electrostatic latent image is formed on a photoreceptor by uniformly charging the photoreceptor and then discharging selected areas of the uniform charge to yield an electrostatic charge pattern corresponding to the desired image (a “latent image”).
After the latent image is formed, charged toner particles are brought into the vicinity of the photoreceptor and are attracted to the latent image to develop the latent image into a visible image. Note that the visible image might not be readily visible to the naked eye depending on the composition of the toner particles.
After the latent image is developed into a visible image on the photoreceptor, a suitable receiver is brought into juxtaposition with the visible image. A suitable electric field is applied to transfer the toner particles of the visible image to the receiver to form the desired print image on the receiver. The imaging process is typically repeated many times with reusable photoreceptors. The photoreceptor is typically in the form of a drum or a roller, but can also be in the form of a belt. The receiver can also be an intermediate transfer member, from which the visible image is further transferred to the final receiver such as a piece of paper. Thermal transfer processes are also useful in the same manner.
The receiver is then removed from its operative association with the photoreceptor and subjected to heat or pressure to permanently fix (“fuse”) the print image to the receiver. Plural print images, e.g., of separations of different colors, are overlaid on one receiver before fusing to form a multi-color print image on the receiver.
The present invention describes improvements to the development or toning process. Numerous methods of development of the latent electrostatic image with charged toner particles are available. Liquid development with insulating carrier fluids including suspended charged toner particles can be used, as can methods with dry toner particles. Common dry toning processes include both mono-component and two-component methods. Mono-component toning systems generally apply dry toner particles to a development roller by way of a foam roller, a doctor blade, or both; the development roller then presents the charged toner to the electrostatic latent image on the photoreceptor. Two-component toning systems typically include toner particles and oppositely charged magnetic carrier particles, the mixture of which is called a two-component developer, attracted to a magnetic brush toning apparatus which then supplies developer to the latent electrostatic image.
Two-component development processes utilizing magnetic brush toning assemblies are also commercially practiced in a variety of forms. What is defined herein as “conventional” two-component development devices utilize a type of magnetic brush roller including a conductive, non-magnetic rotating shell or sleeve with internal stationary magnets. The shell is typically roughened in some fashion to aid in developer transport including flutes or grooves or simple random textures. The magnets are positioned at appropriate places to attract developer from a feed auger or feed roller, and at a position in opposition to the photoreceptor to provide a development zone where the carrier particles are held back magnetically while toner particles are attracted to the latent electrostatic image on the photoreceptor. There can be a blade or skive between the feed auger or roller and the toning roller to regulate the mass area density of the toning nap, the portion of the magnetic brush where the brush is in contact with the photoreceptor surface. The magnet configuration in the region after the toning zone (in the direction of shell rotation) is such that the developer is not attracted to the roller and can fall back into a return auger or a mixing sump depending on the design of the apparatus. Fresh replenisher toner is added to the mixing sump or the feed roller where it can triboelectrically charge against the magnetic carrier particles though mechanical agitation. Three auger toning stations are also common.
The earliest copiers and printers with conventional two-component development processes used magnetic carrier particles of relatively high magnetic saturation moment (Ms) such as sponge iron or stainless steel. These materials have a very low degree of permanent magnetic character; they do not retain a magnetization after exposure to a magnetic field. They have low remanence magnetization (Mr), low magnetic coercivity (Hc) values, and are termed soft magnetic materials. These particles form long, stiff magnetic chains on the toning roller. The mean particle diameter of such materials was typically in the range of 100-250 microns. Controlled electrical conductivity of the developer was important to uniformly tone both large solid areas and lines or text information characterized by high fringe electric fields. The development gap, defined as the closest distance between the toning roller and the photoreceptor, was typically about 200 mil (about 5000 microns). Such methods have been termed “thick nap” development processes.
More recent electrophotographic hardware is characterized by the use of “thin nap” two-component development methods including stationary magnetic poles in the development roller. These processes typically use magnetically soft, ferrite based carrier materials, such as copper-zinc ferrite, manganese ferrite, manganese-magnesium-strontium ferrite, magnetite, and others. The mean diameter of the carrier particles is generally in the range of 20 to 100 microns. The saturation magnetic moment of these ferrites is lower than the materials used for thick nap development processes. The development spacing is generally in the range of 10 to 20 mil, or about 250 to 500 microns. Due to the soft magnetic nature of such carrier materials there is not a particle to particle magnetic interaction in the absence of an external field. The developers are thus free flowing powders in the mixing and transport portions of the development hardware, which generally include simple spiral auger devices. The free flowing nature of soft ferrite developer is advantageous in the avoidance of toner depletion related mixing uniformity artifacts on prints due to rapid mixing and tribocharging with replenisher toner.
U.S. Pat. No. 5,595,850 to Honjo et al. describes magnetically soft manganese-magnesium-strontium ferrite particle compositions and their use as carriers for the purpose of electrophotographic two-component development. Such carriers can also contain a resin coating. The assignee to this patent, Powdertech Co. Ltd. of Chiba-ken, Japan, currently supplies these materials to the electrophotographic copier and printer industry, marketed as the “EF” ferrite product line. Such materials are manufactured over a range of sizes, shapes and electrical conductivities. It is believed that this product line today represents the world's predominant magnetic carrier used in thin nap two-component electrophotographic processes.
Thin nap development processes are also practiced with rotating rather than stationary core magnets. A toning roller with rotating core magnets requires the use of magnetically hard carrier particles such that the alternating magnetic field due to the magnet core rotation causes flipping or jumping action of the developer. A magnetically hard material can retain its magnetization after exposure to a magnetic field; hard magnetic materials are also known as permanent magnetic materials. It has been observed in our laboratory that soft magnetic materials will flow for a short period of time on a toning roller with a rotating magnetic core, but will then start to aggregate into non-moving chains of developer which grow in the circumferential direction of the roller. This aggregation or “freezing” process results in a non-functional magnetic brush. A toning process with rotating core magnets in the magnetic brush roller and permanently magnetized hard magnetic carrier materials is termed small particle development (SPD). The developer on an SPD toning roller transports in response to both the rotation of the magnetic core and the rotation of the shell.
The flow driven by the magnets is in the opposite direction to the rotation of the magnets; if the rotation of the magnets is for example clockwise, the developer appears to jump backwards in the counter-clockwise direction as attracted by each incoming pole of the rotating core magnets. The flow due to core magnet rotation alone can be enough to provide adequate development of toner. However in most applications of SPD development the shell is also rotated, typically in the direction of flow due to the core magnets which is typically co-current with the rotation of the photoreceptor, which means that the shell and its internal magnetic core are rotating in opposite directions. The observed developer motion is thus due to a combination of core driven and shell driven flow. SPD development has been practiced with both rotating and stationary toning shells. The development spacing is typically in the range of 10 to 20 mil, or 250 to 500 microns. Strontium ferrite based carrier particles in the size range of 15 to 30 microns median diameter have been used. In general, as typically practiced, SPD two-component carrier particles are smaller than conventional two-component carrier particles.
Commonly-assigned U.S. Pat. No. 4,473,029 to Fritz et al., the disclosure of which is incorporated herein by reference, describes particular embodiments of SPD toning. Saturation or magnetization of the carrier in a magnetic field that yields an induced moment of at least 25 emu/g is described as useful to reduce the attraction of carrier to the photoreceptor (also known as DPU for developer pick-up).
Commonly-assigned U.S. Pat. No. 4,546,060 to Miskinis et al., the disclosure of which is incorporated herein by reference, describes hard ferrite materials useful as carrier materials for two-component development processes utilizing rotating core magnets. A useful range of coercivity of greater than 300 gauss when magnetically saturated, and of induced moment of greater than 20 emu/g at an applied field of 1000 gauss is described. Magnetization of the carrier by exposing it to a high magnetic field prior to use in the electrophotographic development process is taught. The use of strontium ferrite as a carrier particle meeting these requirements is disclosed.
Commonly-assigned U.S. Pat. No. 5,083,166 to Hill et al., the disclosure of which is incorporated herein by reference, describes a low cost development system which uses the principles of SPD and has its own supply of toner with the entire development subsystem replaced when the toner is depleted. This development subsystem has an irregularly shaped stationary shell surrounding a rotatable magnetic core. The shell is shaped to move hard magnetic carrier through a path which provides a relatively long development zone as well as strong magnetic field strength as the developer moves away from the development zone to avoid DPU in the image. This development subsystem has been used commercially in a single-color electrophotographic printer.
Commonly-assigned U.S. Pat. No. 4,714,046 to Steele, et al., the disclosure of which is incorporated herein by reference, describes a magnetic brush applicator for use in an electrographic reproduction apparatus for applying a magnetic two-component developer to an imaging member including a cylindrical non-magnetic toning shell having a rotatably driven magnetic core positioned therein. The axis of rotation of the magnetic core is displaced from the sleeve axis, such displacement being toward a toning zone at which the applicator applies developer to the imaging member. As a result of the non-concentric arrangement between the toning shell and the magnetic core, the torque requirements for rotating the magnetic core are reduced, developer removal from the toning shell is facilitated, and less thermal energy is introduced into the developer during rotation of the magnetic core.
Eastman Kodak currently manufactures electrophotographic equipment utilizing strontium ferrite based developers, rotating magnetic core toning rollers, and a non-concentric arrangement of the magnetic core and sleeve axes. The NexPress color printer is such a product, and can be used to demonstrate the advantages of the present invention.
The SPD two-component development process including rotating core magnets and hard magnetic carrier materials has particular advantages over two-component conventional development processes utilizing stationary core magnets and soft magnetic carrier materials. The SPD toning zone is characterized by developer flipping and churning action. This enables development at very high speeds with a single toning roller. It is believed that the high rate of development is due to developer motion, resulting from the rotating core magnets, transporting net charged carrier particles that have given up some toner particles to the toning shell where they can become discharged and thus not provide an electrical field which opposes development. This motion also constantly provides fresh developer that has not been depleted of toner to the photoreceptor surface. The conductivity of the hard magnetic carrier can be increased to further enhance the rate of development. The fluidized nature in the toning zone leads to particularly smooth, non-grainy deposits of toner. Directionality effects are also reduced. SPD development is particularly capable of handling a wide range of toner particle sizes.
Both SPD and conventional two-component development processes deposit toner on the photoreceptor at a rate proportional to the electric field in the development zone. The strength of the electric field available to attract toner is determined by the potential difference between the latent image on the photoreceptor and the bias voltage applied to the toning shell, divided by the toning zone spacing or toning shell to photoreceptor distance. In most electrophotographic devices, including the Kodak NexPress, the imaging member which carries the photoreceptor and the toning shell are cylinders. Due to the achievable tolerances during their manufacture these components are not perfectly round. The term runout is used to describe how far out of round a cylinder might be; there are numerous specifically defined engineering runout metrics that can be used to describe out of round cylinders. Cylinders can have one lobe, or be egg shaped with two lobes, and the runout can be non-uniform over the length of the cylinder. Surface runout of the cylinder can also be caused by mounting the cylinder on gudgeons which are not perfectly round or do not have an axis of rotation that is perfectly centered in the gudgeon. As an illustration, simple peak to valley runout values of 1 mil are possible in either imaging member or toning shell cylinders, and the toning zone spacing can typically be 15 mil. The electric field for development is thus modulated by 1/15 or 6.7% due to each of these cylinders as they rotate through the toning zone; both cylinders contribute simultaneously to the variation of the development spacing. The resulting toner density is varied by this continuous changing of toning spacing, and typically can be modulated by about the same 6.7% due to each cylinder. The spatial period of such non-uniformity in the resulting toner image is thus dependent on the rotational speed of the rollers and their diameters. There is thus the need to reduce the spacing sensitivity of two-component development processes.
The nature of SPD developer bulk powder is quite cohesive due to the magnetic interaction between permanently magnetized carrier particles. This clumpy nature can lead to slower mixing with freshly replenished toner, which can lead to image non-uniformity defects known as depletion streaks when the job stream includes very high coverage documents causing a large amount of replenisher toner to be added over a short time. The cohesive nature of the SPD developer requires that special designs be used for transport and mixing of the material. Simple screw auger conveyor designs that work with soft magnetic developers are not suitable for the transport of permanently magnetized, magnetically hard SPD materials. Even with these special designs, the transport and mixing of SPD materials in the SPD development subsystem requires significant energy input. There is thus a need to reduce the bulk powder cohesiveness and accordingly increase the free flow ability of SPD developer materials to improve the mixing of carrier with toner. U.S. Pat. No. 6,617,089 to Meyer et al. discloses an electrographic two-component dry developer composition where carrier particles include a soft magnetic material which has a coercivity of less than 300 gauss when magnetically saturated, a magnetic remanence of less than 20 emu/g when in an applied field of 1000 gauss, and a hard magnetic material with a coercivity of at least 300 gauss when magnetically saturated and an induced moment of at least 20 emu/g when in an applied field of 1000 gauss. The mixture of hard and soft carrier particles is disclosed to be a blend of separate particles. The particular usefulness of such a mixture in a development process with rotating core magnets is not described.
U.S. Pat. No. 6,677,098 to Meyer et al. discloses an electrographic two-component dry developer composition where carrier particles includes a soft magnetic material which has a coercivity of at least 300 gauss when magnetically saturated, exhibit a magnetic moment of less than 20 emu/g when in an applied field of 1000 gauss, and a hard magnetic material with a coercivity of at least 300 gauss when magnetically saturated and an induced moment of at least 20 emu/g when in an applied field of 1000 gauss. The mixture of hard and soft carrier particles is disclosed to be a blend of separate particles, as well as a composite of both types of materials in the same particle. The particular usefulness of such a mixture in a development process with rotating core magnets is not described.
Hard magnetic materials from which to prepare carrier particles appropriate for use in an SPD rotating core development process include magnetoplumbite phase ferrites having the general formula PO.6Fe2O3 wherein P is selected from the group consisting of strontium, barium, calcium, lead, and mixtures thereof. Strontium is particularly useful. These materials are also called hexagonal ferrites. The magnetoplumbite ferrite crystal phase has uniaxial magnetic anisotropy in that the a and b crystallographic axes are paramagnetic, while the c crystallographic axis is ferrimagnetic. Details about the magnetic properties of such ferrite magnetic materials can be found in Ferro-Magnetic Materials, E. P. Wolfarth editor, Elsevier Science Publishers B. V., 1982 and Magnetism and Magnetic Materials, D. Jiles, Chapman and Hall, 1991. Powdertech Co. Ltd. Japan provides strontium ferrite grades known as FCS-150, FCS-200 and FCS-300 to Eastman Kodak Co. for use in the NexPress electrophotographic printer. These materials have volume average particle sizes of approximately 17, 21 microns and 30 microns, respectively. The manufacturing method results in approximately spheroidal particles with a rough texture due to the protrusion of the randomly oriented platelet shaped crystals. The c-axis is the direction perpendicular to the platelet. The crystals are on the order of less than one to a few microns in size, and appear to be uniformly or randomly oriented within a carrier particle as seen in a scanning electron micrograph. This random orientation of the permanently magnetic c-axes within each carrier particle results in some c-axes being magnetized more than others after exposure to the high magnetizing field used in the carrier manufacturing process, since the degree to which a given c-axis will be magnetized is proportional to the magnitude of the field it sees which is dependent on its orientation to that applied magnetizing field. Crystallites whose c-axis is aligned perpendicular to the magnetizing field will not become permanently magnetized. After the bulk carrier powder is subject to the magnetizing field, each particle will thus be a permanent magnet with a net north-south axis.
Soft magnetic powders useful as carrier materials for stationary core two component development processes include copper-zinc ferrite, manganese ferrite, manganese-magnesium-strontium ferrite, and lithium ferrite, among others. Powdertech Co. Ltd. Japan provides grades of manganese-magnesium-strontium ferrite known as EF-35 and EF-20, which have volume average particles sizes of approximately 35 microns and 20 microns, respectively. These particles have a surface which has both smooth and rough textured areas, the roughness due to protruding crystallites as seen in a scanning electron micrograph. Materials including EF ferrite, lithium ferrite, manganese ferrite and copper-zinc ferrite have a spinel crystal structure with cubic magnetic anisotropy. Cubic magnetic anisotropy results in an essentially uniform magnetization response to an applied magnetic field for a given crystallite whatever the angle of that crystallite to the field may be.
The carrier materials used commercially in both conventional and SPD two-component development processes typically have a resinous coating applied. A coating is used for a variety of purposes including controlling the rate and degree of triboelectric charging of the carrier with the toner, controlling that tribocharge with respect to environmental conditions such as temperature and humidity, preventing filming of toner ingredients onto the carrier surface, prolonging the useful life of the developer with regard to the triboelectric charging ability, and changing the effective conductivity of the carrier particle, among others. A wide variety of coating materials have been used commercially as carrier coatings, particularly useful have been silicones, acrylics and fluoropolymers. U.S. Pat. No. 4,935,326, U.S. Pat. No. 4,937,166, and U.S. Pat. No. 5,002,846, all to Creature and Hsu, describe blends of resins particularly useful as carrier coatings for two-component development processes.